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Oxidation Behavior of Zirconium Diboride-Silicon Carbide Composites at High Temperatures.

Karlsdottir, Sigrun N.

Karlsdottir, Sigrun N.

2007

Abstract: The ZrB2-SiC composite is a prominent member of Ultra-High Temperature
Ceramics (UHTCs). Here the oxidation behavior of ZrB2-SiC composites at temperatures
between 1500-1900ºC is studied. The structure and composition of complex oxide scales,
formed at these temperatures, are characterized using microstructural and elemental
analysis.
A novel method, called the Ribbon Method, was developed for testing UHTCs at
high temperatures, rapidly at low cost. Self-supported UHTC ribbon specimens are
resistively heated with a table-top apparatus to achieve temperatures from 900-2000ºC.
The Ribbon Method is a novel method for rapid oxidation characterization of UHTC at
xxi
high temperatures and a valuable alternative to the current high temperature facilities for
UHTCs. Oxidation studies with the Ribbon Method showed that a SiO2 rich borosilicate
surface layer forms during the oxidation of the ZrB2-SiC composite and acts as a
protective barrier at lower temperatures by hindering oxygen diffusion through the
surface layer. The SiO2-rich surface layer starts to volatilize extensively at temperatures
above 1700ºC resulting in a decreases in the oxidation resistance of the composite.
A novel mechanism is proposed for the high temperature oxidation of ZrB2-SiC
based composites. This mechanism is based on liquid transport of oxide liquid solution
formed during oxidation at temperatures around 1550ºC. Patterns in borosilicate surface
layer of oxidized ZrB2-SiC composites were discovered, showing evidence of liquid flow
in the oxide film. These patterns, called here convection cells, are formed when a fluid
B2O3-rich borosilicate liquid containing dissolved ZrO2 is transported to the surface
where the B2O3 is lost by evaporation, depositing ZrO2 in a viscous SiO2-rich liquid. The
driving force for the liquid transport is proposed to be the large volume increase upon
oxidation. Liquid transport of the oxide liquid solution is claimed to play a significant
role in the formation of oxide scales of ZrB2-SiC composites and other boride-SiC based
UTHC composites. Mass transport by liquid flow has not yet been reported for the high
temperature oxidation of ZrB2-SiC composites thus a novel mechanism for the high
temperature oxidation of these materials is presented.